Method and apparatus for tailoring carrier power requirements according to availability in layered modulation systems

ABSTRACT

A method and apparatus transmitting a layered modulation signal having a first signal layer having first signal symbols and a second signal layer having second signal symbols is disclosed. The method comprises the steps of determining a first signal layer modulation carrier power C L  at least in part according to a first signal layer clear sky margin M L  and a first signal layer availability, determining an second signal layer modulation carrier power C U  at least in part according to an second signal layer clear sky margin M U  and an second signal layer availability, modulating the first signal symbols according to a first carrier at the determined first signal layer modulation carrier power; modulating the second signal symbols according to a second carrier at the determined second signal layer modulation carrier power, and transmitting the two layers independently.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims benefit of U.S. Provisional Patent ApplicationNo. 60/421,333, entitled “REDUCING AVAILABILITY TO MITIGATE CARRIERPOWER REQUIREMENTS IN LAYERED MODULATION,” by Ernest C. Chen, Paul R.Anderson and Joseph Santoru, filed Oct. 25, 2002, which application ishereby incorporated by reference herein.

This application is also a continuation-in-part of the followingco-pending and commonly assigned patent application(s), all of whichapplications are incorporated by reference herein:

Utility application Ser. No. 09/844,401, filed Apr. 27, 2001, by ErnestC. Chen, entitled “LAYERED MODULATION FOR DIGITAL SIGNALS,” attorneys'docket number PD-200181 (109.5 1-US-01)

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to systems and methods for transmittingdata, and in particular to a system and method for tailoring carrierpower requirements in a layered modulation system.

2. Description of the Related Art

Digital signal communication systems have been used in various fields,including digital TV signal transmission, either terrestrial orsatellite. As the various digital signal communication systems andservices evolve, there is a burgeoning demand for increased datathroughput and added services. However, it is more difficult toimplement either improvement in old systems and new services when it isnecessary to replace existing legacy hardware, such as transmitters andreceivers. New systems and services are advantaged when they can utilizeexisting legacy hardware. In the realm of wireless communications, thisprinciple is further highlighted by the limited availability ofelectromagnetic spectrum. Thus, it is not possible (or at least notpractical) to merely transmit enhanced or additional data at a newfrequency.

The conventional method of increasing spectral capacity is to move to ahigher-order modulation, such as from quadrature phase shift keying(QPSK) to eight phase shift keying (8PSK) or sixteen quadratureamplitude modulation (16QAM). Unfortunately, QPSK receivers cannotdemodulate conventional 8PSK or 16QAM signals. As a result, legacycustomers with QPSK receivers must upgrade their receivers in order tocontinue to receive any signals transmitted with an 8PSK or 16QAMmodulation.

It is advantageous for systems and methods of transmitting signals toaccommodate enhanced and increased data throughput without requiringadditional spectrum. In addition, it is advantageous for enhanced andincreased throughput signals for new receivers to be backwardscompatible with legacy receivers. There is further an advantage forsystems and methods which allow transmission signals to be upgraded froma source separate from the legacy transmitter.

It has been proposed that a layered modulation signal, transmittingnon-coherently both upper and lower layer signals, can be employed tomeet these needs. Such layered modulation systems allow higherinformation throughput with backwards compatibility. However, even whenbackward compatibility is not required (such as with an entirely newsystem), layered modulation can still be advantageous because itrequires a TWTA peak power significantly lower than that for aconventional 8PSK or 16QAM modulation format for a given throughput.

However, a significant roadblock associated with implementing layeredmodulation is the requirement for satellite transponder powers levelsthat are significantly higher than those currently deployed for givenEarth coverage area.

Accordingly, there is a need for systems and methods for implementinglayered modulation systems at lower transponder power levels. Thepresent invention meets this need and provides further advantages asdetailed hereafter.

SUMMARY OF THE INVENTION

To address the requirements described above, the present inventiondiscloses a method and apparatus transmitting a layered modulationsignal having a first signal layer having first signal symbols and asecond signal layer having second signal symbols. The method comprisesthe steps of determining a first signal layer modulation carrier powerC_(L) at least in part according to a first signal layer clear skymargin M_(L) and a first signal layer availability, determining ansecond signal layer modulation carrier power C_(U) at least in partaccording to an second signal layer clear sky margin M_(U) and an secondsignal layer availability, modulating the first signal symbols accordingto a first carrier at the determined first signal layer modulationcarrier power; modulating the second signal symbols according to asecond carrier at the determined second signal layer modulation carrierpower to generate the layered modulation signal, and transmitting thelayered modulation signal. In one embodiment, the second signal layerclear sky margin is less than the first signal layer clear sky marginwhen the first signal layer availability and the second signal layeravailability are substantially equal. In another embodiment, the secondsignal layer availability is greater than the first signal layeravailability and the second signal layer clear sky margin M_(U) equals$\frac{{\frac{\beta_{U}}{\alpha_{U}}\beta_{U}} + {\beta_{L}T_{L}}}{\alpha_{L} + {\beta_{L}T_{L}}},$wherein α_(U) at least partially represents the rain attenuation of thesecond modulation carrier, α_(L) at least partially represents the rainattenuation of the first layer modulation carrier, β_(U) at leastpartially represents the additional noise in the second modulationcarrier due to rain, and β_(L) at least partially represents theadditional noise in the first modulation carrier due to rain.

BRIEF DESCRIPTION OF THE DRAWINGS

Referring now to the drawings in which like reference numbers representcorresponding parts throughout:

FIG. 1 is a diagram illustrating an overview of a single satellite videodistribution system;

FIG. 2 is a block diagram showing a typical uplink configuration for asingle satellite transponder;

FIG. 3A is a diagram of a representative data stream;

FIG. 3B is a diagram of a representative data packet;

FIG. 4 is a block diagram showing one embodiment of the modulator;

FIG. 5 is a block diagram of an integrated receiver/decoder;

FIGS. 6A-6C are diagrams illustrating the basic relationship of signallayers in a layered modulation transmission;

FIGS. 7A-7C are diagrams illustrating a signal constellation of a secondtransmission layer over the first transmission layer after first layerdemodulation;

FIG. 8 is a diagram showing a system for transmitting and receivinglayered modulation signals;

FIG. 9 is a block diagram depicting one embodiment of an enhancedreceiver/decoder capable of receiving layered modulation signals;

FIG. 10A is a block diagram of one embodiment of the enhancedtuner/modulator and FEC encoder;

FIG. 10B depicts another embodiment of the enhanced tuner/modulatorwherein layer subtraction is performed on the received layered signal;

FIGS. 11A and 11B depicts the relative power levels of exampleembodiments of the present invention;

FIG. 12 illustrates an exemplary computer system that could be used toimplement selected modules or functions the present invention;

FIG. 13 is a diagram showing both upper and lower signal layer clear skymargins as a function of lower layer threshold and desired availability;

FIG. 14 is an illustration showing exemplary lower and upper signallayer clear sky margins as power levels (dB) relative to thermal noisein clear sky conditions;

FIG. 15 is a plot showing the clear sky margin as a function ofunavailability of the upper signal layer; and

FIG. 16 is a diagram illustrating exemplary method steps that can beused to practice one embodiment of the invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

In the following description, reference is made to the accompanyingdrawings which form a part hereof, and which show, by way ofillustration, several embodiments of the present invention. It isunderstood that other embodiments may be utilized and structural changesmay be made without departing from the scope of the present invention.

Video Distribution System

FIG. 1 is a diagram illustrating an overview of a single satellite videodistribution system 100. The video distribution system 100 comprises acontrol center 102 in communication with an uplink center 104 via aground or other link 114 and with a subscriber receiver station 10 via apublic switched telephone network (PSTN) or other link 120. The controlcenter 102 provides program material (e.g. video programs, audioprograms and data) to the uplink center 104 and coordinates with thesubscriber receiver stations 110 to offer, for example, pay-per-view(PPV) program services, including billing and associated decryption ofvideo programs.

The uplink center 104 receives program material and program controlinformation from the control center 102, and using an uplink antenna 106and transmitter 105, transmits the program material and program controlinformation to the satellite 108. The satellite receives and processesthis information, and transmits the video programs and controlinformation to the subscriber receiver station 110 via downlink 118using transmitter 107. The subscriber receiving station 110 receivesthis information using the outdoor unit (ODU) 112, which includes asubscriber antenna and a low noise block converter (LNB).

In one embodiment, the subscriber receiving station antenna is an18-inch slightly oval-shaped Ku-band antenna. The slight oval shape isdue to the 22.5 degree offset feed of the LNB (low noise blockconverter) which is used to receive signals reflected from thesubscriber antenna. The offset feed positions the LNB out of the way soit does not block any surface area of the antenna minimizing attenuationof the incoming microwave signal.

The video distribution system 100 can comprise a plurality of satellites108 in order to provide wider terrestrial coverage, to provideadditional channels, or to provide additional bandwidth per channel. Inone embodiment of the invention, each satellite comprises 16transponders to receive and transmit program material and other controldata from the uplink center 104 and provide it to the subscriberreceiving stations 110. Using data compression and multiplexingtechniques the channel capabilities, two satellites 108 working togethercan receive and broadcast over 150 conventional (non-HDTV) audio andvideo channels via 32 transponders.

While the invention disclosed herein will be described with reference toa satellite-based video distribution system 100, the present inventionmay also be practiced with terrestrial-based transmission of programinformation, whether by broadcasting means, cable, or other means.Further, the different functions collectively allocated among thecontrol center 102 and the uplink center 104 as described above can bereallocated as desired without departing from the intended scope of thepresent invention.

Although the foregoing has been described with respect to an embodimentin which the program material delivered to the subscriber 122 is video(and audio) program material such as a movie, the foregoing method canbe used to deliver program material comprising purely audio informationor other data as well.

Uplink Configuration

FIG. 2 is a block diagram showing a typical uplink configuration for asingle satellite 108 transponder, showing how video program material isuplinked to the satellite 108 by the control center 102 and the uplinkcenter 104. FIG. 2 shows three video channels (which could be augmentedrespectively with one or more audio channels for high fidelity music,soundtrack information, or a secondary audio program for transmittingforeign languages), a data channel from a program guide subsystem 206and computer data information from a computer data source 208.

The video channels are provided by a program source of video material200A-200C (collectively referred to hereinafter as video source(s) 200).The data from each video program source 200 is provided to an encoder202A-202C (collectively referred to hereinafter as encoder(s) 202). Eachof the encoders accepts a program time stamp (PTS) from the controller216. The PTS is a wrap-around binary time stamp that is used to assurethat the video information is properly synchronized with the audioinformation after encoding and decoding. A PTS time stamp is sent witheach I-frame of the MPEG encoded data.

In one embodiment of the present invention, each encoder 202 is a secondgeneration Motion Picture Experts Group (MPEG-2) encoder, but otherdecoders implementing other coding techniques can be used as well. Thedata channel can be subjected to a similar compression scheme by anencoder (not shown), but such compression is usually either unnecessary,or performed by computer programs in the computer data source (forexample, photographic data is typically compressed into *.TIF files or*.JPG files before transmission). After encoding by the encoders 202,the signals are converted into data packets by a packetizer 204A-204F(collectively referred to hereinafter as packetizer(s) 204) associatedwith each source 200.

The data packets are assembled using a reference from the system clock214 (SCR), and from the conditional access manager 210, which providesthe SCD to the packetizers 204 for use in generating the data packets.These data packets are then multiplexed into serial data andtransmitted.

Broadcast Data Stream Format and Protocol

FIG. 3A is a diagram of a representative data stream. The first packetsegment 302 comprises information from video channel 1 (data comingfrom, for example, the first video program source 200A). The next packetsegment 304 comprises computer data information that was obtained, forexample from the computer data source 208. The next packet segment 306comprises information from video channel 5 (from one of the videoprogram sources 200). The next packet segment 308 comprises programguide information such as the information provided by the program guidesubsystem 206. As shown in FIG. 3A, null packets 310 created by the nullpacket module 310 may be inserted into the data stream as desired.

The data stream therefore comprises a series of packets from any one ofthe data sources in an order determined by the controller 216. The datastream is encrypted by the encryption module 218, modulated by themodulator 220 (typically using a QPSK modulation scheme), and providedto the transmitter 222, which broadcasts the modulated data stream on afrequency bandwidth to the satellite via the antenna 106. The receiver500 receives these signals, and using the SCID, reassembles the packetsto regenerate the program material for each of the channels.

FIG. 3B is a diagram of a data packet. Each data packet (e.g. 302-316)is 130 bytes long, and comprises a number of packet segments. The firstpacket segment 320 comprises two bytes of information containing theSCID and flags. The SCID is a unique 12-bit number that uniquelyidentifies the data packet's data channel. The flags include 4 bits thatare used to control other features. The second packet segment 322 ismade up of a 4-bit packet type indicator and a 4-bit continuity counter.The packet type identifies the packet as one of the four data types(video, audio, data, or null). When combined with the SCID, the packettype determines how the data packet will be used. The continuity counterincrements once for each packet type and SCID. The next packet segment324 comprises 127 bytes of payload data, which in the cases of packets302 or 306 is a portion of the video program provided by the videoprogram source 200. The final packet segment 326 is data required toperform forward error correction.

FIG. 4 is a block diagram showing one embodiment of the modulator 220.The modulator 220 optionally comprises a forward error correction (FEC)encoder 404 which accepts the first signal symbols 402 and addsredundant information that are used to reduce transmission errors. Thecoded symbols 405 are modulated by modulator 406 according to a firstcarrier 408 to produce an upper layer modulated signal 410. Secondsymbols 420 are likewise provided to an optional second FEC encoder 422to produce coded second symbols 422. The coded second symbols 422 areprovided to a second modulator 414, which modulates the coded secondsignals according to a second carrier 416 to produce a lower layermodulated signal 418. The resulting signals are then transmitted by oneor more transmitters 420, 422. The upper layer modulated signal 410 andthe lower layer modulated signal 418 are therefore uncorrelated, and thefrequency range used to transmit each layer can substantially orcompletely overlap the frequency spectrum used to transmit the other.For example, as shown in FIG. 4, the frequency spectrum ƒ₁→ƒ₃ 432 of theupper layer signal 410 may overlap the frequency spectrum ƒ₂→ƒ₄ 434 ofthe lower layer signal 418 in frequency band ƒ₂-ƒ₃ 436. The upper layersignal 410, however, must be a sufficiently greater amplitude signalthan the lower layer signal 418, in order to maintain the signalconstellations shown in FIG. 6 and FIG. 7. The modulator 220 may alsoemploy pulse shaping techniques (illustrated by pulse p(t) 430) toaccount for the limited channel bandwidth. Although FIG. 4 illustratesthe same pulse shaping p(t) 430 being applied to both layers, differentpulse shaping can be applied to each layer as well.

Integrated Receiver/Decoder

FIG. 5 is a block diagram of an integrated receiver/decoder (IRD) 500(also hereinafter alternatively referred to as receiver 500). Thereceiver 500 comprises a tuner/demodulator 504 communicatively coupledto an ODU 112 having one or more LNBs 502. The LNB 502 converts the12.2-to 12.7 GHz downlink 118 signal from the satellites 108 to, e.g., a950-1450 MHz signal required by the IRD's 500 tuner/demodulator 504. TheLNB 502 may provide either a dual or a single output. The single-outputLNB 502 has only one RF connector, while the dual output LNB 502 has twoRF output connectors and can be used to feed a second tuner 504, asecond receiver 500, or some other form of distribution system.

The tuner/demodulator 504 isolates a single, digitally modulated 24 MHztransponder, and converts the modulated data to a digital data stream.Further details regarding the demodulation of the received signalfollow.

The digital data stream is then supplied to a forward error correction(FEC) decoder 506. This allows the IRD 500 to reassemble the datatransmitted by the uplink center 104 (which applied the forward errorcorrection to the desired signal before transmission to the subscriberreceiving station 110) verifying that the correct data signal wasreceived, and correcting errors, if any. The error-corrected data may befed from the FEC decoder module 506 to the transport module 508 via an8-bit parallel interface.

The transport module 508 performs many of the data processing functionsperformed by the IRD 500. The transport module 508 processes datareceived from the FEC decoder module 506 and provides the processed datato the video MPEG decoder 514 and the audio MPEG decoder 517. In oneembodiment of the present invention, the transport module, video MPEGdecoder and audio MPEG decoder are all implemented on integratedcircuits. This design promotes both space and power efficiency, andincreases the security of the functions performed within the transportmodule 508. The transport module 508 also provides a passage forcommunications between the microcontroller 510 and the video and audioMPEG decoders 514, 517. As set forth more fully hereinafter, thetransport module also works with the conditional access module (CAM) 512to determine whether the subscriber receiving station 110 is permittedto access certain program material. Data from the transport module canalso be supplied to external communication module 526.

The CAM 512 functions in association with other elements to decode anencrypted signal from the transport module 508. The CAM 512 may also beused for tracking and billing these services. In one embodiment of thepresent invention, the CAM 512 is a smart card, having contactscooperatively interacting with contacts in the IRD 500 to passinformation. In order to implement the processing performed in the CAM512, the IRD 500, and specifically the transport module 508 provides aclock signal to the CAM 512.

Video data is processed by the MPEG video decoder 514. Using the videorandom access memory (RAM) 536, the MPEG video decoder 514 decodes thecompressed video data and sends it to an encoder or video processor 516,which converts the digital video information received from the videoMPEG module 514 into an output signal usable by a display or otheroutput device. By way of example, processor 516 may comprise a NationalTV Standards Committee (NTSC) or Advanced Television Systems Committee(ATSC) encoder. In one embodiment of the invention both S-Video andordinary video (NTSC or ATSC) signals are provided. Other outputs mayalso be utilized, and are advantageous if high definition programming isprocessed.

Audio data is likewise decoded by the MPEG audio decoder 517. Thedecoded audio data may then be sent to a digital to analog (D/A)converter 518. In one embodiment of the present invention, the D/Aconverter 518 is a dual D/A converter, one for the right and leftchannels. If desired, additional channels can be added for use insurround sound processing or secondary audio programs (SAPs). In oneembodiment of the invention, the dual D/A converter 518 itself separatesthe left and right channel information, as well as any additionalchannel information. Other audio formats may similarly be supported. Forexample, other audio formats such as multi-channel DOLBY DIGITAL AC-3may be supported.

A description of the processes performed in the encoding and decoding ofvideo streams, particularly with respect to MPEG and JPEGencoding/decoding, can be found in Chapter 8 of “Digital TelevisionFundamentals,” by Michael Robin and Michel Poulin, McGraw-Hill, 1998,which is hereby incorporated by reference herein.

The microcontroller 510 receives and processes command signals from theremote control 524, an IRD 500 keyboard interface, and/or another inputdevice. The microcontroller receives commands for performing itsoperations from a processor programming memory, which permanently storessuch instructions for performing such commands. The processorprogramming memory may comprise a read only memory (ROM) 538, anelectrically erasable programmable read only memory (EEPROM) 522 or,similar memory device. The microcontroller 510 also controls the otherdigital devices of the IRD 500 via address and data lines (denoted “A”and “D” respectively, in FIG. 5).

The modem 540 connects to the customer's phone line via the PSTN port120. It calls, e.g. the program provider, and transmits the customer'spurchase information for billing purposes, and/or other information. Themodem 540 is controlled by the microprocessor 510. The modem 540 canoutput data to other I/O port types including standard parallel andserial computer I/O ports.

The present invention also comprises a local storage unit such as thevideo storage device 532 for storing video and/or audio data obtainedfrom the transport module 508. Video storage device 532 can be a harddisk drive, a read/writable compact disc of DVD, a solid state RAM, orany other storage medium. In one embodiment of the present invention,the video storage device 532 is a hard disk drive with specializedparallel read/write capability so that data may be read from the videostorage device 532 and written to the device 532 at the same time. Toaccomplish this feat, additional buffer memory accessible by the videostorage 532 or its controller may be used. Optionally, a video storageprocessor 530 can be used to manage the storage and retrieval of thevideo data from the video storage device 532. The video storageprocessor 530 may also comprise memory for buffering data passing intoand out of the video storage device 532. Alternatively or in combinationwith the foregoing, a plurality of video storage devices 532 can beused. Also alternatively or in combination with the foregoing, themicrocontroller 510 can also perform the operations required to storeand or retrieve video and other data in the video storage device 532.

The video processing module 516 input can be directly supplied as avideo output to a viewing device such as a video or computer monitor. Inaddition, the video and/or audio outputs can be supplied to an RFmodulator 534 to produce an RF output and/or 8 vestigial side band (VSB)suitable as an input signal to a digital terrestrial television tuner.This allows the receiver 500 to operate with televisions without a videooutput.

Each of the satellites 108 comprises a transponder, which acceptsprogram information from the uplink center 104, and relays thisinformation to the subscriber receiving station 110. Known multiplexingtechniques are used so that multiple channels can be provided to theuser. These multiplexing techniques include, by way of example, variousstatistical or other time domain multiplexing techniques andpolarization multiplexing. In one embodiment of the invention, a singletransponder operating at a single frequency band carries a plurality ofchannels identified by respective service channel identification (SCID).

Preferably, the IRD 500 also receives and stores a program guide in amemory available to the microcontroller 510. Typically, the programguide is received in one or more data packets in the data stream fromthe satellite 108. The program guide can be accessed and searched by theexecution of suitable operation steps implemented by the microcontroller510 and stored in the processor ROM 538. The program guide may includedata to map viewer channel numbers to satellite transponders and servicechannel identifications (SCIDs), and also provide TV program listinginformation to the subscriber 122 identifying program events.

The functionality implemented in the IRD 500 depicted in FIG. 5 can beimplemented by one or more hardware modules, one or more softwaremodules defining instructions performed by a processor, or a combinationof both.

The present invention provides for the modulation of signals atdifferent power levels and advantageously for the signals to benon-coherent from each layer. In addition, independent modulation andcoding of the signals may be performed. Backwards compatibility withlegacy receivers, such as a quadrature phase shift keying (QPSK)receiver is enabled and new services are provided to new receivers. Atypical new receiver of the present invention uses two demodulators andone remodulator as will be described in detail hereafter.

In a typical backwards-compatible embodiment of the present invention,the legacy QPSK signal is boosted in power to a higher transmission (andreception) level. The legacy receiver will not be able to distinguishthe new lower layer signal from additive white Gaussian noise and thusoperates in the usual manner. The optimum selection of the layer powerlevels is based on accommodating the legacy equipment, as well as thedesired new throughput and services.

The combined layered signal is demodulated and decoded by firstdemodulating the upper layer to remove the upper carrier. The stabilizedlayered signal may then have the upper layer FEC decoded and the outputupper layer symbols communicated to the upper layer transport. The upperlayer symbols are also employed in a remodulator, to generate anidealized upper layer signal. The idealized upper layer signal is thensubtracted from the stable layered signal to reveal the lower layersignal. The lower layer signal is then demodulated and FEC decoded andcommunicated to the lower layer transport.

The new lower layer signal is provided with a sufficient carrier tothermal noise ratio to function properly. The new lower layer signal andthe boosted legacy signal are non-coherent with respect to each other.Therefore, the new lower layer signal can be implemented from adifferent TWTA and even from a different satellite. The new lower layersignal format is also independent of the legacy format, e.g., it may beQPSK or 8PSK, using the conventional concatenated FEC code or using anew Turbo code. The lower layer signal may even be an analog signal.

Signals, systems and methods using the present invention may be used tosupplement a pre-existing transmission compatible with legacy receivinghardware in a backwards-compatible application or as part of apreplanned layered modulation architecture providing one or moreadditional layers at a present or at a later date.

Layered Signals

FIGS. 6A-6C illustrate the basic relationship of signal layers in alayered modulation transmission. In these figures the horizontal axis isfor the in-phase, or “T” value of the displayed symbol, and the verticalaxis for the quadratue, or “Q” value of the displayed symbol. FIG. 6Aillustrates a first layer signal constellation 600 of a transmissionsignal showing the signal points or symbols 602. This signalconstellation is FIG. 6B illustrates the second layer signalconstellation of symbols 604 over the first layer signal constellation600 where the layers are coherent. FIG. 2C illustrates a second signallayer 606 of a second transmission layer over the first layerconstellation where the layers may be non-coherent. The second layer 606rotates about the first layer constellation 602 due to the relativemodulating frequencies of the two layers in a non-coherent transmission.Both the first and second layers rotate about the origin due to thefirst layer modulation frequency as described by path 608.

FIGS. 7A-7C are diagrams illustrating a signal constellation of a secondtransmission layer over the first transmission layer after first layerdemodulation. FIG. 7A shows the constellation 700 before the firstcarrier recovery loop (CRL) and FIG. 7B shows the constellation 704after CRL. In this case, the signal points of the second layer areactually rings 702. FIG. 7C depicts a phase distribution of the receivedsignal with respect to nodes 602.

Relative modulating frequencies cause the second layer constellation torotate around the nodes of the first layer constellation. After thesecond layer CRL this rotation is eliminated. The radius of the secondlayer constellation is determined by its power level. The thickness ofthe rings 702 is determined by the carrier to noise ratio (CNR) of thesecond layer. As the two layers are non-coherent, the second layer mayalso be used to transmit analog or digital signals.

FIG. 8 is a diagram showing a system for transmitting and receivinglayered modulation signals. Separate transmitters 107A, 107B, as may belocated on any suitable platform, such as satellites 108A, 108B, areused to non-coherently transmit different layers of a signal of thepresent invention. Uplink signals are typically transmitted to eachsatellite 108A, 108B from one or more transmitters 105 via an antenna106. The layered signals 808A, 808B (downlink signals) are received atreceiver antennas 112A, 112B, such as satellite dishes, each with a lownoise block (LNB) 810A, 810B where they are then coupled to integratedreceiver/decoders (IRDs) 500, 802. Because the signal layers may betransmitted non-coherently, separate transmission layers may be added atany time using different satellites 108A, 108B or other suitableplatforms, such as ground based or high altitude platforms. Thus, anycomposite signal, including new additional signal layers will bebackwards compatible with legacy receivers 500, which will disregard thenew signal layers. To ensure that the signals do not interfere, thecombined signal and noise level for the lower layer must be at or belowthe allowed noise floor for the upper layer.

Layered modulation applications include backwards compatible andnon-backwards compatible applications. “Backwards compatible” in thissense, describes systems in which legacy receivers 500 are not renderedobsolete by the additional signal layer(s). Instead, even if the legacyreceivers 500 are incapable of decoding the additional signal layer(s),they are capable of receiving the layered modulated signal and decodingthe original signal layer. In these applications, the pre-existingsystem architecture is accommodated by the architecture of theadditional signal layers. “Non-backwards compatible” describes a systemarchitecture which makes use of layered modulation, but the modulationand coding scheme employed is such that pre-existing equipment isincapable of receiving and decoding the information on additional signallayer(s).

The pre-existing legacy IRDs 500 decode and make use of data only fromthe layer (or layers) they were designed to receive, unaffected by theadditional layers. The present invention may be applied to existingdirect satellite services which are broadcast to individual users inorder to enable additional features and services with new receiverswithout adversely affecting legacy receivers and without requiringadditional signal frequencies.

Demodulator and Decoder

FIG. 9 is a block diagram depicting one embodiment of an enhanced IRD802 capable of receiving layered modulation signals. The enhanced IRD802 includes a feedback path 902 in which the FEC decoded symbols arefed back to a enhanced modified tuner/demodulator 904 and transportmodule 908.

FIG. 10A is a block diagram of one embodiment of the enhancedtuner/modulator 904 and FEC encoder 506. FIG. 10A depicts receptionwhere layer subtraction is performed on a signal where the upper carrierhas been demodulated. The upper layer of the received combined signal1016 from the LNB 502, which may contain legacy modulation format, isprovided to and processed by an upper layer demodulator 1004 to producethe stable demodulated signal 1020. The demodulated signal 1020 is fedto a communicatively coupled FEC decoder 1002 which decodes the upperlayer to produce the upper layer symbols which are output to an upperlayer transport. The upper layer symbols are also used to generate anidealized upper layer signal. The upper layer symbols may be producedfrom the decoder 1002 after Viterbi decode (BER<10⁻³ or so) or afterReed-Solomon (RS) decode (BER<10⁻⁹ or so), in typical decodingoperations known to those skilled in the art. The upper layer symbolsare provided via feedback path 902 from the upper layer decoder 1002 toa remodulator 1006 and then a module which applies the distortion thatwould be introduced by the satellite downlink network. This effectivelyproduces an idealized upper layer signal. The idealized upper levelsignal is subtracted from the demodulated upper layer signal 1020.

In order for the subtraction to leave a clean lower layer signal, theupper layer signal must be precisely reproduced. The modulated signalmay have been distorted, for example, by traveling wave tube amplifier(TWTA) non-linearity or other non-linear or linear distortions in thetransmission channel. The distortion effects are estimated from thereceived signal after the fact or from TWTA characteristics which may bedownloaded into the IRD in AM-AM and/or AM-PM maps 1014.

A subtractor 1012 then subtracts the idealized upper layer signal fromthe stable demodulated signal 1020. This leaves the lower-power secondlayer signal. The subtractor 1012 may include a buffer or delay functionto retain the stable demodulated signal 1020 while the idealized upperlayer signal is being constructed. The second layer signal isdemodulated by the lower level demodulator 1010 and FEC decoded bydecoder 1008 according to its signal format to produce the lower layersymbols, which are provided to a transport module similar to 508 but forthe lower layer.

FIG. 10B depicts another embodiment wherein layer subtraction isperformed on the received layered signal. In this case, the upper layerdemodulator 1004 produces the upper carrier signal 1022. An uppercarrier signal 1022 is provided to the remodulator 1006. The remodulator1006 provides the remodulated signal to the non-linear distortion mapper1018 which effectively produces an idealized upper layer signal. Unlikethe embodiment shown in FIG. 10A, in this embodiment, the idealizedupper layer signal includes the upper layer carrier for subtraction fromthe received combined signal 416.

Other equivalent methods of layer subtraction will occur to thoseskilled in the art and the present invention should not be limited tothe examples provided here. Furthermore, those skilled in the art willunderstand that the present invention is not limited to two layers;additional layers may be included. Idealized upper layers are producedthrough remodulation from their respective layer symbols and subtracted.Subtraction may be performed on either the received combined signal or ademodulated signal. Finally, it is not necessary for all signal layersto be digital transmissions; the lowest layer may be an analogtransmission.

The following analysis describes the exemplary two layer demodulationand decoding. It will be apparent to those skilled in the art thatadditional layers may be demodulated and decoded in a similar manner.The incoming combined signal is represented as:${s_{UL}(t)} = {{f_{U}\left( {M_{U}\quad{\exp\left( {{j\quad\omega_{U}t} + \theta_{U}} \right)}\quad{\sum\limits_{m = {- \infty}}^{\infty}{S_{Um}{p\left( {t - {mT}} \right)}}}} \right)} + {f_{L}\left( {M_{L}\quad{\exp\left( {{j\quad\omega_{L}t} + \theta_{L}} \right)}\quad{\sum\limits_{m = {- \infty}}^{\infty}{S_{Lm}{p\left( {t - {mT} + {\Delta\quad T_{m}}} \right)}}}} \right)} + {n(t)}}$where, M_(U) is the magnitude of the upper layer QPSK signal and M_(L)is the magnitude of the lower layer QPSK signal and M_(L)<<M_(U). Thesignal frequencies and phase for the upper and lower layer signals arerespectively ω_(U), θ_(U) and ω_(U), θ_(U), respectively. The symboltiming misalignment between the upper and lower layers is ΔT_(m). Theexpression p(t−mT) represents the time shifted version of the pulseshaping filter p(t) 430 employed in signal modulation. QPSK symbolsS_(Um) and S_(Lm) are elements of$\left\{ {{\exp\left( {j\quad\frac{n\quad\pi}{2}} \right)},\quad{n = 0},1,2,3} \right\}.$ƒ_(U)(·) and ƒ_(L)(·) denote the distortion function of the TWTAs forthe respective signals.

Ignoring ƒ_(U)(·) and ƒ_(L)(·) and noise n(t), the following representsthe output of the demodulator 1004 to the FEC decoder 1002 afterremoving the upper carrier:${s_{UL}^{\prime}(t)} = {{M_{U}\quad{\sum\limits_{m = {- \infty}}^{\infty}{S_{Um}{p\left( {t - {mT}} \right)}}}} + {M_{L}\quad\exp\left\{ {{{j\left( {\omega_{L} - \omega_{U}} \right)}t} + \theta_{L} - \theta_{U}} \right\}\quad{\sum\limits_{m = {- \infty}}^{\infty}{S_{Lm}{p\left( {t - {mT} + {\Delta\quad T_{m}}} \right)}}}}}$Because of the magnitude difference between M_(U) and M_(L), the upperlayer decoder 402 disregards the M_(L) component of the s′_(UL)(t).

After subtracting the upper layer from s_(UL)(t) in the subtractor 1012,the following remains:${s_{L}(t)} = {M_{L}\quad\exp\left\{ {{{j\left( {\omega_{L} - \omega_{U}} \right)}t} + \theta_{L} - \theta_{U}} \right\}\quad{\sum\limits_{m = {- \infty}}^{\infty}{S_{Lm}{p\left( {t - {mT} + {\Delta\quad T_{m}}} \right)}}}}$Any distortion effects, such as TWTA nonlinearity effects are estimatedfor signal subtraction. In a typical embodiment of the presentinvention, the upper and lower layer frequencies are substantiallyequal. Significant improvements in system efficiency can be obtained byusing a frequency offset between layers.

Using the present invention, two-layered backward compatible modulationwith QPSK doubles the current legacy system capacity that uses a legacyoperating mode with a 6/7 FEC code rate. This capacity increase isenabled by transmitting a backward compatible upper layer carrierthrough a TWTA that is approximately 6.2 dB above the power used in thelegacy system. The new lower layer QPSK signals may be transmitted froma separate transmitter, or from a different satellite for example.

Systems using 16QAM modulation could be designed to provide similartransmission capacity, but this modulation format requires reasonablylinear transmitting amplifiers. With layered modulation, separateamplifiers can be used for each layer, and if QPSK signals are used forthese layers then these amplifiers can be used in a more efficientnon-linear mode. Thus layered modulation eliminates the need for lessefficient linear travelling wave tube amplifiers (TWTAS) as are neededfor 16QAM. Also, no phase error penalty is imposed on higher ordermodulations such as 8PSK and 16QAM.

Backward Compatible Applications

FIG. 11A depicts the relative power levels 1100 of example embodimentsof the present invention without taking into account the effects ofrain. Accommodation of rain fade effects comes through the inclusion ofclear sky margin in the calculation of transmit power levels, and thisis treated in a later section. FIG. 11A is not a scale drawing. Thisembodiment doubles the pre-existing rate 6/7 capacity by using a TWTAwhose power level is 6.2 dB above a pre-existing (legacy) TWTA, and asecond TWTA whose power level is 2 dB below that of a pre-existing(legacy) TWTA. This embodiment uses upper and lower QPSK layers whichare non-coherent. An FEC code rate of 6/7 is also used for both layers.In this embodiment, the signal of the legacy QPSK signal 1102 is used togenerate the upper layer 1104 and a new QPSK layer is the lower layer1110. The legacy QPSK signal 1102 has a threshold CNR (i.e., the carrierto noise ratio required to achieve acceptable performance) ofapproximately 7 dB. The new lower QPSK layer 1110 has a threshold CNR ofapproximately 5 dB. In the present invention, then, the lower QPSK layertransmit power level 1110 is first set so that the received lower layerpower is 5 dB above the reference thermal noise power level 1108. Boththe thermal noise and the lower layer signal will appear as noise to theupper layer legacy QPSK signal, and this combined noise power must betaken into account when setting the upper layer transmit power level.The combined power of these two noise sources 1106 is 6.2 dB above thereference thermal noise floor 1108. The legacy QPSK signal must then beboosted in power by approximately 6.2 dB above the legacy signal powerlevel 1102 bringing the new power level to approximately 13.2 dB as theupper layer 1104. In this way the combined lower layer signal power andthermal noise power is kept at or below the tolerable noise floor 1106of the upper layer. It should be noted that the invention may beextended to multiple layers with mixed modulations, coding and coderates.

In an alternate embodiment of this backwards compatible application, anFEC code rate of 2/3 may be used for both the upper and lower layers1104, 1110. In this case, the threshold CNR of the legacy QPSK signal1102 (with an FEC code rate of 2/3) is approximately 5.8 dB. The legacysignal 1102 is boosted by approximately 5.3 dB to approximately 11.1 dB(4.1 dB above the legacy QPSK signal 1102 with an FEC code rate of 2/3)to form the upper QPSK layer 1104. The new lower QPSK layer 1110 has athreshold CNR of approximately 3.8 dB. The total signal and noise of thelower layer 1110 is kept at or below approximately 5.3 dB, the tolerablenoise floor 1106 of the upper QPSK layer. In this case, the totalcapacity is 1.55 times that the legacy signal 1102.

In a further embodiment of a backwards compatible application of thepresent invention the code rates between the upper and lower layers1104, 1110 may be mixed. For example, the legacy QPSK signal 502 may beboosted by approximately 5.3 dB to approximately 12.3 dB with the FECcode rate unchanged at 6/7 to create the upper QPSK layer 1104. The newlower QPSK layer 1110 may use an FEC code rate of 2/3 with a thresholdCNR of approximately 3.8 dB. In this case, the total capacity is 1.78times that of the legacy signal 1102.

Non-Backward Compatible Applications

As previously discussed the present invention may also be used in“non-backward compatible” applications. In a first example embodiment,two QPSK layers 1104, 1110 are used each at an FEC code rate of 2/3. Theupper QPSK layer 504 has a threshold CNR of approximately 4.1 dB aboveits noise floor 1106 and the lower QPSK layer 1110 also has a thresholdCNR of approximately 4.1 dB. The combined power of the thermal noise andthe lower QPSK layer 1110 is approximately 5.5 dB above the referencethermal noise level 1108. The CNR for the upper QPSK signal 1104 is thenset at approximately 9.6 dB (4.1+5.5 dB), merely 2.4 dB above the legacyQPSK signal rate 6/7. The capacity is then a factor of approximately1.56 compared to the legacy rate 6/7.

FIG. 11B depicts the relative power levels of an alternate embodimentwherein both the upper and lower layers 1104, 1110 can be below thelegacy signal level 1102. The two QPSK layers 1104, 1110 use a code rateof 1/2. The lower and upper QPSK layers have a threshold CNR ofapproximately 2.0 dB. In this case, the upper QPSK layer 1104 isapproximately 2.0 dB above its noise floor 1106 of approximately 4.1 dB.The upper layer signal level of 6.1 dB is lower than the 7.0 dB for thelegacy signal. The capacity of this embodiment is a factor ofapproximately 1.17 compared to the legacy rate 6/7.

Hardware Environment

FIG. 12 illustrates an exemplary computer system 1200 that could be usedto implement selected modules or functions the present invention. Thecomputer 1202 comprises a processor 1204 and a memory, such as randomaccess memory (RAM) 1206. The computer 1202 is operatively coupled to adisplay 1222, which presents images such as windows to the user on agraphical user interface 1218B. The computer 1202 may be coupled toother devices, such as a keyboard 1214, a mouse device 1216, a printer,etc. Of course, those skilled in the art will recognize that anycombination of the above components, or any number of differentcomponents, peripherals, and other devices, may be used with thecomputer 1202.

Generally, the computer 1202 operates under control of an operatingsystem 1208 stored in the memory 1206, and interfaces with the user toaccept inputs and commands and to present results through a graphicaluser interface (GUI) module 1218A. Although the GUI module 1218A isdepicted as a separate module, the instructions performing the GUIfunctions can be resident or distributed in the operating system 1208,the computer program 1210, or implemented with special purpose memoryand processors. The computer 1202 also implements a compiler 1212 whichallows an application program 1210 written in a programming languagesuch as COBOL, C++, FORTRAN, or other language to be translated intoprocessor 1204 readable code. After completion, the application 1210accesses and manipulates data stored in the memory 1206 of the computer1202 using the relationships and logic that was generated using thecompiler 1212. The computer 1202 also optionally comprises an externalcommunication device such as a modem, satellite link, Ethernet card, orother device for communicating with other computers.

In one embodiment, instructions implementing the operating system 1208,the computer program 1210, and the compiler 1212 are tangibly embodiedin a computer-readable medium, e.g., data storage device 1220, whichcould include one or more fixed or removable data storage devices, suchas a zip drive, floppy disc drive 1224, hard drive, CD-ROM drive, tapedrive, etc. Further, the operating system 1208 and the computer program1210 are comprised of instructions which, when read and executed by thecomputer 1202, causes the computer 1202 to perform the steps necessaryto implement and/or use the present invention. Computer program 1210and/or operating instructions may also be tangibly embodied in memory1206 and/or data communications devices 1230, thereby making a computerprogram product or article of manufacture according to the invention. Assuch, the terms “article of manufacture,” “program storage device” and“computer program product” as used herein are intended to encompass acomputer program accessible from any computer readable device or media.

Those skilled in the art will recognize many modifications may be madeto this configuration without departing from the scope of the presentinvention. For example, those skilled in the art will recognize that anycombination of the above components, or any number of differentcomponents, peripherals, and other devices, may be used with the presentinvention.

Using the techniques described herein, as will be shown later, the clearsky margin required for the upper signal layer 402 is considerably lessthan the clear sky margin that would be required if the signal were sentby itself. It is also considerably less than that required for the lowersignal layer 420. In a rain fade condition, the upper and lower layersfade together. Thus, the primary source of noise for the upper signallayer 402 fades as fast as the upper layer signal itself, allowing for asignificantly reduced upper layer clear sky margin. The presentinvention takes full advantage of this effect. Conversely, the clear skymargin required for the lower layer must be set high enough to accountfor the fade of the lower layer carrier relative to its primary sourceof noise, thermal noise, which increases in rain. Hence, the requiredclear sky margin for the upper signal layer 402 can be reduced whencompared to that which is required for the lower signal layer 420.Alternatively or in combination, the technique described below can beused to design a layered modulation system that provides higheravailability levels for the upper layer than for the lower layer.

Clear Sky Margin

The distribution of power to the upper and lower layer carriersdiscussed earlier (FIGS. 11A and 11B) did not consider the affects ofrain attenuation on the upper and lower layer signals. These affects canbe large, acting to both decrease the desired signal level and toincrease the noise level. In the case of layered modulation theseeffects must be carefully considered for each layer. Additional power isadded to each layer to accommodate these rain effects, and this addedpower is called clear sky margin (CSM). In the technique describedbelow, the design of the upper signal layer 410 takes advantage of thefact that both the lower signal layer 418 and the upper signal layer 410are attenuated by an equal amount in a rain fade condition. Also, sincethe upper signal layer 410 must be locked and reconstructed before thelower signal layer 418 can be accurately demodulated, the lower signallayer 418 can be no more “available” in a statistical sense than theupper signal layer 410. In a critical condition where the upper andlower signal layers have exactly the same availability, both signalswill drop to their respective operating thresholds simultaneously whenthe rain attenuation reaches a sufficient value.

Upper and Lower Signal Layers with Equal Availability

Assuming that the clear sky thermal noise level is represented by N, andthat the carrier-to-noise threshold level is given by T_(L) and T_(U),for the lower and upper signal layers 418, 410, respectively, thethreshold levels T_(L) and T_(U) can be defined in a number of ways. Forpurposes of illustration, the analysis that follows assumes that thethreshold levels T_(L) and T_(U) are quasi-error-free thresholds. Thisis the operating point where the number of bit errors detected at theoutput of the forward error correction decoder 506 have dropped to aboutone error per hour or one error per day.

Assuming for the moment that there is a given link availabilityrequirement, from that value, suitable values for rain attenuation andrain noise can be determined. Defining a parameter α to represent theamount of rain attenuation present (α<=1), and β to represent theincrease in noise due to atmospheric rain (β>=1), both of which are afunction of the desired signal availability, the lower signal layer 418carrier power C_(L) required to provide the necessary link availabilitycan be determined from the expression: $\begin{matrix}{T_{L} = \frac{\alpha\quad C_{L}}{\beta_{N}}} & {{Equation}\quad(1)}\end{matrix}$Solving for C_(L): $\begin{matrix}{C_{L} = \frac{\beta\quad N\quad T_{L}}{\alpha}} & {{Equation}\quad(2)}\end{matrix}$

The values α and β are both a function of the desired availability, andare typically defined through the use of rain attenuation models thatwould be readily known to someone skilled in the art.

A clear sky margin (defined as a ratio between the clear sky carrier tonoise-plus-interference ratio and the threshold carrier tonoise-plus-interference ratio) can be computed for each layer. For thelower signal layer 418, the clear sky margin M_(L) becomes:$\begin{matrix}{M_{L} = {\frac{C_{L}}{N\quad T_{L}} = \frac{\beta}{\alpha}}} & {{Equation}\quad(3)}\end{matrix}$The upper signal layer 410 carrier power C_(U) necessary to provide therequired link availability is computed by noting that when the uppersignal layer carrier is at a threshold condition, the carrier isattenuated by the factor α. The noise term however, contains both thelink thermal noise power (increased by the rain) and the lower levelcarrier power (attenuated by rain). Consequently, the upper layercarrier power C_(L) necessary to provide the required link availabilityis defined by the Equation (4) below: $\begin{matrix}{T_{U} = \frac{\alpha\quad C_{U}}{{\beta\quad N} + {\alpha\quad C_{L}}}} & {{Equation}\quad(4)}\end{matrix}$

Using this expression, the required upper level carrier power C_(U) isdescribed in Equation (5) below. $\begin{matrix}{C_{\upsilon} = \frac{\left( {{\beta\quad N} + {\alpha\quad C_{L}}} \right)T_{U}}{\alpha}} & {{Equation}\quad(5)}\end{matrix}$

And the clear sky margin for the upper signal layer 410 becomes$\begin{matrix}\begin{matrix}{M_{U} = {\frac{C_{U}}{N + C_{L}}\frac{1}{T_{U}}}} \\{= {\frac{C_{U}}{N + C_{L}}\frac{{\beta\quad N} + {\alpha\quad C_{L}}}{\alpha\quad C_{U}}}} \\{= \frac{\left( {\frac{C_{L}}{N} + \frac{\beta}{\alpha}} \right)}{\left( {\frac{C_{L}}{N} + 1} \right)}}\end{matrix} & {{Equation}\quad(6)}\end{matrix}$

Noting that $\begin{matrix}{\frac{C_{L}}{N} = \frac{\beta\quad T_{L}}{\alpha}} & {{Equation}\quad(7)}\end{matrix}$

The upper signal layer clear sky margin can be written in terms of thelower signal layer threshold as shown in Equation (8) below.$\begin{matrix}{M_{U} = {\frac{\frac{\beta}{\alpha}\left( {T_{L} + 1} \right)}{\frac{\beta}{\alpha}\left( {T_{L} + \frac{\alpha}{\beta}} \right)} = \frac{\left( {T_{L} + 1} \right)}{\left( {T_{L} + \frac{\alpha}{\beta}} \right)}}} & {{Equation}\quad(8)}\end{matrix}$

In a typical application, the values of α might change from −1 to −5 dBand the values for β might range from 2 to 4 dB, depending on thedesired availability. Since the lower level clear sky margin is (β-α),when expressed in dB, then it can be seen that typical lower signallayer clear sky margins will range from 3 to 9 dB, depending on thedesired availability.

It would ordinarily be expected that the lower signal layer clear skymargin would also be required for the upper signal layer, which wouldrequire very high transmitter powers. However, this is not necessarybecause the upper and lower signal layers fade together in rain, asshown in the derivation for the upper layer clear sky margin in Equation(4) above. Hence, the upper layer clear sky margin depends on thecarrier to noise threshold and to a lesser extent on the ratio of α toβ. The required upper layer clear sky margin is typically 1 dB or less,and approaches 0 dB with increasing lower signal layer 420carrier-to-noise threshold.

FIG. 13 is a diagram showing both upper and lower signal layer clear skymargins as a function of lower layer threshold and desired availability.Plot 1302 shows the lower layer clear sky margin as a function of thelower signal layer carrier-to-noise ratio threshold for a lower signallayer availability of 99.95%. Plots 1304-1308 show the same for lowersignal layer availabilities of 99.90%, 99.85%, and 99.80%, respectively.Plots 1310-1316 show the upper layer clear sky margin for upper signallayer availabilities of 99.95%, 99.90%, 99.85%, and 99.80% respectively.Note in this figure that the upper layer clear sky margins are much lessthan the lower layer clear sky margins. The smaller clear sky marginsfor the upper layer are critical to the performance of layeredmodulation because it lowers the required satellite transmit power ofthe upper layer carrier.

The ratio of the upper signal layer carrier to thermal noise in clearsky can be computed as a function of α, β, and the upper and lowercarrier-to-noise ratios.

Beginning with the relation $\begin{matrix}{C_{U} = \frac{\left( {{\beta\quad N} + {\alpha\quad C_{L}}} \right)\quad T_{U}}{\alpha}} & {{Equation}\quad(9)}\end{matrix}$

we can obtain $\begin{matrix}{{\frac{C_{U}}{N} = \frac{\left( {\beta + {\alpha\quad\frac{C_{L}}{N}}} \right)T_{U}}{\alpha}}{\frac{C_{U}}{N} = \frac{\left( {\beta + {\alpha\quad\frac{\beta}{\alpha}T_{L}}} \right)\quad T_{U}}{\alpha}}{\frac{C_{U}}{N} = {\frac{\beta}{\alpha}\left( {1 + T_{L}} \right)T_{U}}}} & {{Equation}\quad(10)}\end{matrix}$

If the lower signal layer 418 were not present (e.g. a legacy signal),the required clear sky carrier to noise ratio would not include the term(1+T_(L)). This added term accounts for the presence of the lower signallayer 418 as interfering noise to the upper signal layer 410. Notingthat N refers to the thermal noise only, the total noise plus lowerlayer interference power seen by the upper signal layer demodulator isdominated by the lower layer signal layer carrier power.

Equation (10) provides a minimum value for C_(U) relative to the thermalnoise for both the upper and lower signal layers to exhibit the sameavailability. By increasing C_(U) above this level, the availability ofthe upper signal layer 410 can be increased over that of the lowersignal layer 418.

FIG. 14 is an illustration showing exemplary lower and upper signallayer clear sky margins as power levels (dB) relative to thermal noisein clear sky conditions. In this example, the lower signal layercarrier-to-noise-plus-interference threshold was set at 6.0 dB, and theupper signal layer carrier-to-noise-plus-interference threshold was setto 5.0 dB. The values for α and β are about −2.0 and +3.0 dB,respectively. Note that the lower signal layer threshold point plusclear sky margin give a clear sky lower layer carrier power of 11.0 dBrelative to thermal noise N. The combination of thermal noise and lowersignal layer carrier power is 11.4 dB, which is the noise plusinterference level seen by the upper signal layer carrier.

Summing (in dB) the upper layer required threshold to thenoise-plus-interference value of 5 dB to 11.4 dB gives the upper layerthreshold point of 16.4 dB relative to thermal noise N. The requiredclear sky margin above this point is only 0.6 dB, yet in a rain fadecondition, bot the upper and lower signal layers will exhibit the sameavailability.

Upper and Lower Signal Layer Margins with Improved Upper LayerAvailability

The upper and lower signal layers 410, 418 can be designed withdifferent availability objectives a well. As previously noted, the lowersignal layer 418 availability cannot be better than the upper signallayer 410 availability, since successful demodulation of the lowersignal layer 418 depends on successful demodulation of the upper signallayer 410. However, the upper signal layer 410 can be designed withbetter availability than the lower signal layer 418 by increasing theupper signal layer margin. As demonstrated below, significantimprovements can be made in the upper signal layer 410 availability withonly small increases in the upper signal layer 410 margin. This is asignificant advantage of the non-coherent layered modulation techniquesdescribed herein.

Modifying Equation (1) to differentiate between the parameters α and β,for the upper and lower signal layers yields Equation (11) below.$\begin{matrix}{T_{L} = \frac{\alpha_{L}C_{L}}{\beta_{L}N}} & {{Equation}\quad(11)}\end{matrix}$

This yields Equations (12) and (13). $\begin{matrix}{C_{L} = \frac{\beta_{L}{NT}_{L}}{\alpha_{L}}} & {{Equation}\quad(12)} \\{M_{L} = \frac{\beta_{L}}{\alpha_{L}}} & {{Equation}\quad(13)}\end{matrix}$

For improved availability in the upper layer,α_(U)<α_(L)  Equation (14)andβ_(U)>β_(L).  Equation (15)

Noting that when the upper signal layer 410 is at threshold, the newupper signal layer values for α and β will apply, $\begin{matrix}{T_{U} = {\frac{\alpha_{U}C_{U}}{{\beta_{U}N} + {\alpha_{U}C_{L}}}.}} & {{Equation}\quad(16)}\end{matrix}$

Referring to Equation (5), the new upper signal carrier power becomes$\begin{matrix}{C_{U} = {\frac{\left( {{\beta_{U}N} + {\alpha_{U}C_{L}}} \right)T_{U}}{\alpha_{U}}.}} & {{Equation}\quad(17)}\end{matrix}$

Using Equation (6), the following relationship is derived:$\begin{matrix}\begin{matrix}{M_{U} = {\frac{C_{U}}{N + C_{L}}\frac{1}{T_{U}}}} \\{= {\frac{C_{U}}{N + C_{L}}\frac{{\beta_{U}N} + {\alpha_{U}C_{L}}}{\alpha_{U}C_{U}}}} \\{= {\frac{\left( {\frac{C_{L}}{N} + \frac{\beta_{U}}{\alpha_{U}}} \right)}{\left( {\frac{C_{L}}{N} + 1} \right)}.}}\end{matrix} & {{Equation}\quad(18)}\end{matrix}$

Using, from Equation (12), $\begin{matrix}{\frac{C_{L}}{N} = \frac{\beta_{L}T_{L}}{\alpha_{L}}} & {{Equation}\quad(19)}\end{matrix}$

we obtain, $\begin{matrix}{M_{U} = {\frac{\frac{\beta_{U}}{\alpha_{U}} + \frac{\beta_{L}T_{L}}{\alpha_{L}}}{1 + \frac{\beta_{L}T_{L}}{\alpha_{L}}} = \frac{{\frac{\alpha_{L}}{\alpha_{U}}\beta_{U}} + {\beta_{L}T_{L}}}{\alpha_{L} + {\beta_{L}T_{L}}}}} & {{Equation}\quad(20)}\end{matrix}$

Note that Equation (20) reduces to Equation (8) if the availabilities ofthe upper and lower signal layers are equal (e.g. α_(L)=α_(U) andβ_(L)=β_(U)).

FIG. 15 is a plot of Equation (20) as a function of the unavailabilityof the upper signal layer 410. In this example, the lower levelunavailability is 0.02% (since unavailability is (1-availability), thistranslates to an availability of 99.8%) and the lower signal layerthreshold is 6 dB.

As can be seen in the lower curve of FIG. 15, which plots M_(U), theupper layer clear sky margin defined by (18) or (20), the upper signallayer performance can be improved (e.g. lower unavailability) byincreasing the upper signal layer clear sky margin by only 10ths of adB, as shown in curve 1504. As upper curve 1502 shows, in aconventionally modulated, single-layer satellite link, the clear skymargin would have to be improved by 3 dB to achieve the same performanceimprovement.

Thus, if one of the signal layers requires higher availability than theother, then that layer must be designated as the upper signal layer.Similarly, if backward compatibility is required, then the signal layerthat provides such backward compatibility must be designated as theupper signal layer. Normally, there is no conflict between theserequirements, as the backwards-compatible layer is normally also desiredto be the higher availability layer. If, however, thenon-backwards-compatible layer requires higher availability than thebackwards-compatible layer, a conflicting requirement exists. This canbe resolved by designing the system such that the availability of thesignal layers is equal and at the higher availability value.

FIG. 16 is a diagram illustrating exemplary method steps that can beused to practice one embodiment of the invention. A first signal layermodulation carrier power C_(L) is determined at least in part accordingto a first signal layer clear sky margin M_(L) and a first signal layeravailability, as shown in block 1602. In one embodiment, this isaccomplished by determining the first level carrier power C_(L)according to ${C_{L} = \frac{\beta\quad{NT}_{L}}{\alpha}},$wherein β/α is the first layer clear sky margin M_(L), β comprises avalue representing an increase in noise of the layered modulation signaldue to atmospheric rain, a comprises a value representing rainattenuation of the layered modulation signal, N comprises a valuerepresenting clear-sky thermal noise, and T_(L) comprises a first signallayer carrier-to-noise threshold level. In block 1604, a second signallayer modulation carrier power C_(U) is determined at least in partaccording to a second signal layer clear sky margin M_(U) and a secondsignal layer availability. In one embodiment, this is accomplished bydetermining an second level carrier power C_(U) according to${C_{U} = \frac{\left( {{\beta\quad N} + {\alpha\quad C_{L}}} \right)T_{U}}{\alpha}},$and wherein the second layer clear sky margin$M_{U} = \frac{\left( {T_{L} + 1} \right)}{\left( {T_{L} + \frac{\alpha}{\beta}} \right)}$and T_(U) comprises a second signal layer carrier-to-noise thresholdlevel. Next, the first signal symbols are modulated according to a firstcarrier at the determined first signal layer modulation carrier power,as shown in block 1606. Then the second signal symbols are modulatedaccording to a second carrier at the determined second signal layermodulation carrier power, as shown in block 1608. The modulated firstand second signals are then transmitted independently to the satellite,as shown in block 1610.

In one embodiment wherein the first signal layer availability and thesecond signal availability are substantially equal (e.g. α_(L)≈α_(U) andβ_(L)≈β_(U)), the second signal layer clear sky margin M_(U) is lessthan the first signal layer clear sky margin M_(L). In anotherembodiment, the second signal layer availability is greater than thefirst signal layer availability (α_(U)<α_(L) and β_(U)>β_(L), forexample), and the second signal layer clear sky margin M_(U) equals$\frac{{\frac{\beta_{U}}{\alpha_{U}}\beta_{U}} + {\beta_{L}T_{L}}}{\alpha_{L} + {\beta_{L}T_{L}}},$wherein α_(U) at least partially represents the rain attenuation of thesecond modulation carrier, α_(L) at least partially represents the rainattenuation of the first layer modulation carrier, β_(U) at leastpartially represents the additional noise in the second modulationcarrier due to rain, and β_(L) at least partially represents theadditional noise in the first modulation carrier due to rain.

CONCLUSION

This concludes the description of the preferred embodiments of thepresent invention. The foregoing description of the preferred embodimentof the invention has been presented for the purposes of illustration anddescription. It is not intended to be exhaustive or to limit theinvention to the precise form disclosed. Many modifications andvariations are possible in light of the above teaching. For example, itis noted that the uplink configurations depicted and described in theforegoing disclosure can be implemented by one or more hardware modules,one or more software modules defining instructions performed by aprocessor, or a combination of both.

It is intended that the scope of the invention be limited not by thisdetailed description, but rather by the claims appended hereto. Theabove specification, examples and data provide a complete description ofthe manufacture and use of the composition of the invention. Since manyembodiments of the invention can be made without departing from thespirit and scope of the invention, the invention resides in the claimshereinafter appended.

1. A method of transmitting a layered modulation signal having a firstsignal layer having first signal symbols and a second signal layerhaving second signal symbols, comprising the steps of: determining afirst signal layer modulation carrier power C_(L) at least in partaccording to a first signal layer clear sky margin M_(L) and a firstsignal layer availability; determining an second signal layer modulationcarrier power C_(U) at least in part according to an second signal layerclear sky margin M_(U) and an second signal layer availability;modulating the first signal symbols according to a first carrier at thedetermined first signal layer modulation carrier power; modulating thesecond signal symbols according to a second carrier at the determinedsecond signal layer modulation carrier power to generate the layeredmodulation signal; transmitting the modulated first signal symbols andsecond signal symbols; and wherein the second signal layer clear skymargin is less than the first signal layer clear sky margin when thefirst signal layer availability and the second signal layer availabilityare substantially equal.
 2. The method of claim 1, wherein the modulatedfirst signal symbols and the modulated second signal symbols areindependently transmitted.
 3. The method of claim 1, wherein the firstsignal layer is transmitted on a different frequency range than thesecond signal layer.
 4. The method of claim 1, wherein: the step ofdetermining the first signal layer modulation carrier power C_(L) atleast in part according to a first layer clear sky margin M_(L) and afirst layer availability comprises the step of determining a first levelcarrier power C_(L) according to${C_{L} = \frac{\beta\quad{NT}_{L}}{\alpha}},$ wherein β/α is the firstlayer clear sky margin M_(L), β comprises a value representing anincrease in noise of the layered modulation signal due to atmosphericrain, a comprises a value representing rain attenuation of the layeredmodulation signal, N comprises a value representing clear-sky thermalnoise, and T_(L) comprises a first signal layer carrier-to-noisethreshold level; and the step of determining the second signal layermodulation carrier power C_(U) at least in part according to an secondlayer clear sky margin M_(U) and a second layer availability comprisesthe step of determining an second level carrier power C_(U) according to${C_{U} = \frac{\left( {{\beta\quad N} + {\alpha\quad C_{L}}} \right)T_{U}}{\alpha}},$and wherein the second layer clear sky margin$M_{U} = \frac{\left( {T_{L} + 1} \right)}{\left( {T_{L} + \frac{\alpha}{\beta}} \right)}$and T_(U) comprises a second signal layer carrier-to-noise thresholdlevel.
 5. The method of claim 1, wherein: the first signal symbols aremodulated according to a first carrier; the second signal symbols aremodulated according to a second carrier; and wherein the first carrieris randomly phased with respect to the second carrier.
 6. The method ofclaim 5, further comprising the steps of: demodulating and decoding thesecond signal layer to produce the second signal symbols; re-encodingand remodulating the second signal symbols and subtracting there-encoded and remodulated second signal symbols from the layeredmodulation signal to produce the first signal layer; and demodulatingthe first signal layer to produce the first signal symbols.
 7. A methodof transmitting a layered modulation signal having a first signal layerhaving first signal symbols and a second signal layer having secondsignal symbols, comprising the steps of: determining a first signallayer modulation carrier power C_(L) at least in part according to afirst signal layer clear sky margin M_(L) and a first signal layeravailability; determining an second signal layer modulation carrierpower C_(U) at least in part according to an second layer clear skymargin M_(U) and an second signal layer availability; modulating thefirst signal symbols according to a first carrier at the determinedfirst signal layer modulation carrier power; modulating the secondsignal symbols according to a second carrier at the determined secondsignal layer modulation carrier power; transmitting the modulated firstsignal symbols and the modulated second signal symbols; and wherein thesecond signal layer availability is greater than the first signal layeravailability and the second signal layer clear sky margin M_(U) equals$\frac{{\frac{\beta_{U}}{\alpha_{U}}\beta_{U}} + {\beta_{L}T_{L}}}{\alpha_{L} + {\beta_{L}T_{L}}},$wherein α_(U) at least partially represents the rain attenuation of thesecond modulation carrier, α_(L) at least partially represents the rainattenuation of the first layer modulation carrier, β_(U) at leastpartially represents the additional noise in the second modulationcarrier due to rain, and β_(L) at least partially represents theadditional noise in the first modulation carrier due to rain.
 8. Themethod of claim 7, wherein the modulated first signal symbols and themodulated second signal symbols are independently transmitted.
 9. Themethod of claim 7, wherein α_(U)<α_(L) and β_(U)>β_(L).
 10. The methodof claim 7, wherein the first signal layer is transmitted on a differentfrequency range than the second signal layer.
 11. The method of claim 7,wherein: the first signal layer is modulated according to a firstcarrier; the second signal layer is modulated according to a secondcarrier; and wherein the first carrier is randomly phased with respectto the second carrier.
 12. The method of claim 11, further comprisingthe steps of: demodulating the second carrier and decoding the secondlayer to produce the second signal symbols; re-encoding and remodulatingthe second signal symbols and subtracting the recoded and remodulatedsecond signal symbols from the layered modulation signal to produce thefirst signal layer; and demodulating the first carrier and decoding thedemodulated first carrier to produce the first signal symbols.
 13. Anapparatus for transmitting a layered modulation signal having a firstsignal layer having first signal symbols and a second signal layerhaving second signal symbols, comprising: means for determining a firstsignal layer modulation carrier power C_(L) at least in part accordingto a first signal layer clear sky margin M_(L) and a first signal layeravailability; means for determining an second signal layer modulationcarrier power C_(U) at least in part according to an second signal layerclear sky margin M_(U) and an second signal layer availability; meansfor modulating the first signal symbols according to a first carrier atthe determined first signal layer modulation carrier power; means formodulating second signal symbols according to a second carrier at thedetermined second signal layer modulation carrier power to generate thesecond signal layer; means for transmitting the modulated first signalsymbols and the modulated second signal symbols; and wherein the secondsignal layer clear sky margin is less than the first signal layer clearsky margin when the first signal layer availability and the secondsignal layer availability are substantially equal.
 14. The apparatus ofclaim 13, wherein the modulated first signal symbols and the modulatedsecond signal symbols are independently transmitted.
 15. The apparatusof claim 13, wherein the second signal layer modulation is an uppermodulation layer and the first signal layer modulation layer is a lowermodulation layer.
 16. The apparatus of claim 15, wherein: the means fordetermining the first signal layer modulation carrier power C_(L) atleast in part according to a first layer clear sky margin M_(L) and afirst layer availability comprises means for determining a first levelcarrier power C_(L) according to${C_{L} = \frac{\beta\quad{NT}_{L}}{\alpha}},$ wherein β/α is the firstlayer clear sky margin M_(L), β comprises a value representing anincrease in noise of the layered modulation signal due to atmosphericrain, α comprises a value representing rain attenuation of the layeredmodulation signal, N comprises a value representing clear-sky thermalnoise, and T_(L) comprises a first signal layer carrier-to-noisethreshold level; and the means for determining the second signal layermodulation carrier power C_(U) at least in part according to an secondlayer clear sky margin M_(U) and a second layer availability comprisesmeans for determining an second level carrier power C_(U) according to${C_{U} = \frac{\left( {{\beta\quad N} + {\alpha\quad C_{L}}} \right)T_{U}}{\alpha}},$and wherein the second layer clear sky margin$M_{U} = \frac{\left( {T_{L} + 1} \right)}{\left( {T_{L} + \frac{\alpha}{\beta}} \right)}$and T_(U) comprises a second signal layer carrier-to-noise thresholdlevel.
 17. The apparatus of claim 13, wherein: the first signal symbolsare modulated according to a first carrier; the second signal symbolsare modulated according to a second carrier; and wherein the firstcarrier is randomly phased with respect to the second carrier.
 18. Theapparatus of claim 17, further comprising: means for demodulating anddecoding the second signal layer to produce the second signal symbols;means for re-encoding and remodulating the second signal symbols andsubtracting the re-encoded and remodulated second signal symbols fromthe layered modulation signal to produce the first signal layer; andmeans for demodulating and decoding the first signal layer to producethe first signal symbols.
 19. An apparatus for transmitting a layeredmodulation signal having a first signal layer having first signalsymbols and a second signal layer having second signal symbols,comprising: means for determining a first signal layer modulationcarrier power C_(L) at least in part according to a first signal layerclear sky margin M_(L) and a first signal layer availability; means fordetermining an second signal layer modulation carrier power C_(U) atleast in part according to an second layer clear sky margin M_(U) and ansecond signal layer availability; means for modulating the first signalsymbols according to a first carrier at the determined first signallayer modulation carrier power; means for modulating the second signalsymbols according to a second carrier at the determined second signallayer modulation carrier power to generate the second modulated signal;means for transmitting the modulated first signal symbols and themodulated second signal symbols; and wherein the second signal layeravailability is greater than the first signal layer availability and thesecond signal layer clear sky margin${M_{U} = \frac{{\frac{\beta_{U}}{\alpha_{U}}\beta_{U}} + {\beta_{L}T_{L}}}{\alpha_{L} + {\beta_{L}T_{L}}}},$wherein α_(U) at least partially represents the rain attenuation of thesecond modulation carrier, α_(L) at least partially represents the rainattenuation of the first layer modulation carrier, β_(U) at leastpartially represents the additional noise in the second modulationcarrier due to rain, and β_(L) at least partially represents theadditional noise in the first modulation carrier due to rain.
 20. Theapparatus of claim 19, wherein the modulated first signal symbols andthe modulated second signal symbols are independently transmitted. 21.The apparatus of claim 19, wherein α_(U)<α_(L) and β_(U)>β_(L).
 22. Theapparatus of claim 19, wherein the first signal layer is transmitted ona different frequency range than the second signal layer.
 23. Theapparatus of claim 19, wherein: the first signal layer is modulatedaccording to a first carrier; the second signal layer is modulatedaccording to a second carrier; and wherein the first carrier is randomlyphased with respect to the second carrier.
 24. The apparatus of claim23, further comprising: means for demodulating and decoding the secondcarrier and decoding the second layer to produce the second signalsymbols; means for re-encoding and remodulating the second signalsymbols and subtracting the re-encoded remodulated second signal symbolsfrom the layered modulation signal to produce the first signal layer;and means for demodulating the first carrier and decoding thedemodulated first carrier to produce the first signal symbols.
 25. Anapparatus for transmitting a layered modulation signal having a firstsignal layer having first signal symbols and a second signal layerhaving second signal symbols, comprising: a processor for determining afirst signal layer modulation carrier power C_(L) at least in partaccording to a first signal layer clear sky margin M_(L) and a firstsignal layer availability, and for determining an second signal layermodulation carrier power C_(U) at least in part according to an secondsignal layer clear sky margin M_(U) and an second signal layeravailability; a modulator, communicatively coupled to the processor, themodulator for modulating the first signal symbols according to a firstcarrier at the determined first signal layer modulation carrier power; asecond modulator, communicatively coupled to the processor, the secondmodulator for modulating second signal symbols according to a secondcarrier at the determined second signal layer modulation carrier powerto generate the second signal layer; at least one transmitter,communicatively coupled to the modulator and the second modulator, theat least one transmitter for transmitting the modulated first signalsymbols and the modulated second signal symbols; and wherein the secondsignal layer clear sky margin is less than the first signal layer clearsky margin when the first signal layer availability and the secondsignal layer availability are substantially equal.
 26. The apparatus ofclaim 25, wherein the modulated first signal symbols and the modulatedsecond signal symbols are independently transmitted.
 27. The apparatusof claim 25, wherein the second signal layer modulation is an uppermodulation layer and the first signal layer modulation layer is a lowermodulation layer.
 28. The apparatus of claim 27, wherein the processorcomprises: a module for determining a first level carrier power C_(L)according to ${C_{L} = \frac{\beta\quad{NT}_{L}}{\alpha}},$ wherein β/αis the first layer clear sky margin M_(L), β comprises a valuerepresenting an increase in noise of the layered modulation signal dueto atmospheric rain, a comprises a value representing rain attenuationof the layered modulation signal, N comprises a value representingclear-sky thermal noise, and T_(L) comprises a first signal layercarrier-to-noise threshold level; and a second module for determining ansecond level carrier power C_(U) according to${C_{U} = \frac{\left( {{\beta\quad N} + {\alpha\quad C_{L}}} \right)T_{U}}{\alpha}},$and wherein the second layer clear sky margin$M_{U} = \frac{\left( {T_{L} + 1} \right)}{\left( {T_{L} + \frac{\alpha}{\beta}} \right)}$and T_(U) comprises a second signal layer carrier-to-noise thresholdlevel.
 29. The apparatus of claim 25, wherein: the first signal symbolsare modulated according to a first carrier; the second signal symbolsare modulated according to a second carrier; and wherein the firstcarrier is randomly phased with respect to the second carrier.
 30. Theapparatus of claim 29, further comprising: a demodulator fordemodulating the second layer signal; a decoder, communicatively coupledto the decoder, for decoding the demodulated second signal layer toproduce the second signal symbols; a re-encoder, communicatively coupledto the decoder, the re-encoder for re-encoding the second signal symbolsa modulator, communicatively coupled to the re-encoder, the modulatorfor remodulating the re-encoded second signal symbols; a differencer,communicatively coupled to the modulator, for subtracting the re-encodedand remodulated second signal symbols from the layered modulation signalto produce the first signal layer; and a second demodulator, fordemodulating and decoding the first signal layer to produce the firstsignal symbols.
 31. An apparatus for transmitting a layered modulationsignal having a first signal layer having first signal symbols and asecond signal layer having second signal symbols, comprising: aprocessor, for determining a first signal layer modulation carrier powerC_(L) at least in part according to a first signal layer clear skymargin M_(L) and a first signal layer availability, and for determiningan second signal layer modulation carrier power C_(U) at least in partaccording to an second layer clear sky margin M_(U) and an second signallayer availability; a modulator, communicatively coupled to theprocessor, the modulator for modulating the first signal symbolsaccording to a first carrier at the determined first signal layermodulation carrier power; a second modulator, communicatively coupled tothe processor, the second modulator for modulating the second signalsymbols according to a second carrier at the determined second signallayer modulation carrier power to generate the second modulated signal;at least one transmitter, communicatively coupled to the secondmodulator, the second modulator for transmitting the modulated firstsignal symbols and the modulated second signal symbols; and wherein thesecond signal layer availability is greater than the first signal layeravailability and the second signal layer clear sky margin${M_{U} = \frac{{\frac{\beta_{U}}{\alpha_{U}}\beta_{U}} + {\beta_{L}T_{L}}}{\alpha_{L} + {\beta_{L}T_{L}}}},$wherein α_(U) at least partially represents the rain attenuation of thesecond modulation carrier, α_(L) at least partially represents the rainattenuation of the first layer modulation carrier, β_(U) at leastpartially represents the additional noise in the second modulationcarrier due to rain, and β_(L) at least partially represents theadditional noise in the first modulation carrier due to rain.
 32. Theapparatus of claim 31, wherein the modulated first signal symbols andthe modulated second signal symbols are independently transmitted. 33.The apparatus of claim 31, wherein α_(U)<α_(L) and β_(U)>β_(L).
 34. Theapparatus of claim 31, wherein the first signal layer is transmitted ona different frequency range than the second signal layer.
 35. Theapparatus of claim 31, wherein: the first signal layer is modulatedaccording to a first carrier; the second signal layer is modulatedaccording to a second carrier; and wherein the first carrier is randomlyphased with respect to the second carrier.
 36. The apparatus of claim35, further comprising: a demodulator, for demodulating and decoding thesecond carrier and decoding the second layer to produce the secondsignal symbols; a re-encoder, for re-encoding the second signal symbols;a modulator, communicatively coupled to the re-encoder, the modulatorfor re-modulating the second signal symbols; a differencer,communicatively coupled to the modulator, for subtracting the re-encodedremodulated second signal symbols from the layered modulation signal toproduce the first signal layer; and a second demodulator,communicatively coupled to the differencer, the second demodulator fordemodulating the first carrier and decoding the demodulated firstcarrier to produce the first signal symbols.
 37. The apparatus of claim31, wherein the first signal layer is transmitted on a differentfrequency range than the second signal layer.